Why do we need bigger scopes?

[iantaylor2uk]

Let's go back to astro cameras. They have a fixed sensor area, and when you attach them to a telescope you get a field of view (which determines the magnification) which only depends on the focal length of the telescope (which is fixed). If you want to change the magnification you use a reducer or a Barlow. If you use a 0.8x reducer on an f/6 telescope it becomes f/4.8. if you use a 2x Barlow it becomes f/12. Although the focal length of the telescope is fixed, the f ratio of the system has changed.

What I am saying is that if you are a visual observer, changing eyepieces has a similar effect to that for cameras and a higher magnification eyepiece will give a larger f ratio for the system.

 

 

[vlaiv]

7 minutes ago, iantaylor2uk said:

Let's go back to astro cameras. They have a fixed sensor area, and when you attach them to a telescope you get a field of view (which determines the magnification) which only depends on the focal length of the telescope (which is fixed). If you want to change the magnification you use a reducer or a Barlow. If you use a 0.8x reducer on an f/6 telescope it becomes f/4.8. if you use a 2x Barlow it becomes f/12. Although the focal length of the telescope is fixed, the f ratio of the system has changed.

What I am saying is that if you are a visual observer, changing eyepieces has a similar effect to that for cameras and a higher magnification eyepiece will give a larger f ratio for the system.

 

 

If you want to compare camera systems to visual observation - then use afocal method instead

This is analog to visual as we have telescope, eyepiece, camera lens (human eye lens) and sensor (retina).

Now if you want to magnify image - well use different eyepiece, but F/ratio of both camera lens and human eye - nor telescope for that matter will change

[Stu]

1 hour ago, iantaylor2uk said:

Your example is where the object you are viewing is very bright already and extended over many receiving cells in the retina. 

However, my understanding is that telescopes of larger aperture allow you to see fainter objects and I think if you consider pointlike objects such as stars, my explanation would allow you to understand why the larger aperture allow you to see fainter objects. 

Trouble is, extended objects don’t behave like stars which are point sources. My understanding of star visibility is that it is related to the point source maintaining its brightness whilst dimming the surrounding background as mag increases ie increasing the contrast between star and background, whereas an extended object’s contrast doesn’t change with higher mag.

[iantaylor2uk]

53 minutes ago, vlaiv said:

If you want to compare camera systems to visual observation - then use afocal method instead

This is analog to visual as we have telescope, eyepiece, camera lens (human eye lens) and sensor (retina).

Now if you want to magnify image - well use different eyepiece, but F/ratio of both camera lens and human eye - nor telescope for that matter will change

I agree that the focal length of the telescope, the human eye, or the eyepiece, are all fixed values.

However it is the focal length of the "whole system" that is important. If we go back to the example of a camera sensor, it is accepted that field of view is inversely proportional to the focal length of the telescope. As I said before, if you use a 2x Barlow lens to get higher magnification, the effective focal length of the system is twice as long, the field of view is half what it was, and since the aperture of the telescope is the same, the f ratio "of the system" has effectively doubled. If it was f/6 it has now become f/12.

When you use an eyepiece, for visual observing, the same logic holds. If you start off with a 20mm eyepiece, and then move to a 10mm eyepiece, with the higher magnification eyepiece, the field of view has effectively halved (compared to the 20mm eyepiece) and so the effective focal length of the system has also doubled, and since the aperture of the telescope has not changed, the f ratio has also doubled. 

Just to be clear what I am saying here. If you have a telescope with twice the aperture of another telescope, but they have the same f ratio, then 4x as many photons will enter your eye, at each point on your retina. This is why objects are brighter with larger telescopes.  

If you are doing visual observing, and changing eyepieces to change magnification, then you are effectively changing the f ratio "the system" and at higher magnifications, the f ratio will be higher, so acting as a slower lens, which is why objects appear dimmer. 

Edited October 6, 2021 by iantaylor2uk

[iantaylor2uk]

I think the link below is quite useful: 

https://cosmicpursuits.com/943/telescopes-explained/

Edited October 6, 2021 by iantaylor2uk

[vlaiv]

46 minutes ago, iantaylor2uk said:

If you are doing visual observing, and changing eyepieces to change magnification, then you are effectively changing the f ratio "the system" and at higher magnifications, the f ratio will be higher, so acting as a slower lens, which is why objects appear dimmer. 

Yes, but only extended sources - point sources remain of the same brightness.

This is the crux of the question. If you take 8" scope and 4" scope and produce same exit pupil - you should get same brightness of extended object.

On one hand - 8" scope gathers x4 more photons, but on other hand - effective F/ratio as you put it will be twice as high compared to small scope - if you keep exit pupil the same (twice as long FL) - which will make image x4 as large by surface - and hence "dimmer". Two cancel out perfectly. We should see things equally bright in 8" scope and 4" scope.

To put it in astrophotography terms you seem to be familiar with - 8" F/5 scope will be equally fast as 4" F/5 scope - right?

Why do we see faint stuff in 8" F/5 scope that we don't see in 4" F/5 scope when using the same eyepiece?

Edited October 6, 2021 by vlaiv

[iantaylor2uk]

I think it is working out what is the right comparison which is difficult. If you wanted to keep the focal length the same (so the field of view is the same) then you should be comparing an 8" f/5 with a 4" f/10. 

On the other hand if you keep the f ratio the same then the field of view is altered. 

There is also the other fact that larger telescopes tend to have lower f ratios - many refractors in the 80mm to 120mm range are f/6 to f/8 whereas many large (Newtonian) reflectors (250mm +) will be f/4 or f/5, so they are not only better at gathering light but also faster too. 

[Stu]

41 minutes ago, iantaylor2uk said:

I think it is working out what is the right comparison which is difficult. If you wanted to keep the focal length the same (so the field of view is the same) then you should be comparing an 8" f/5 with a 4" f/10. 

On the other hand if you keep the f ratio the same then the field of view is altered. 

There is also the other fact that larger telescopes tend to have lower f ratios - many refractors in the 80mm to 120mm range are f/6 to f/8 whereas many large (Newtonian) reflectors (250mm +) will be f/4 or f/5, so they are not only better at gathering light but also faster too. 

Speed is not really relevant for visual astronomy. At the same magnification an 8” f5 scope will show the same image size and brightness as an 8” f10.

I see it fairly simply. An 8” f5 will show an object at the same size but higher brightness than a 4” f10, (given the same eyepiece) because the exit pupil will be twice as big.

If the focal ratios are the same, the 8” will show the object at the same brightness but twice the size as the 4”, again, assuming the same eyepiece, because the exit pupils will be the same but the magnification in the 4” will be half that in the 8”.

Simples.

[cloudsweeper]

2 hours ago, Stu said:

Trouble is, extended objects don’t behave like stars which are point sources. My understanding of star visibility is that it is related to the point source maintaining its brightness whilst dimming the surrounding background as mag increases ie increasing the contrast between star and background, whereas an extended object’s contrast doesn’t change with higher mag.

Yes - star brightness depends on aperture;  ext ob (EO) brightness depends on exit pupil (and mag, as I suggested before).

But I still see the other part of the above differently to you Stu.  It's my thinking that contrast for all objects increases with mag (to a point), as background brightness drops.  It can get complicated though: a small EO for example is close to being like a point object, so will dim less with mag, thus showing better contrast as background brightness continues to drop.

Doug.

[pete_81]

Interesting thread, thought I'd add my $0.02...

Surely the f/# of an optical system is a measure of how many photons are hitting unit area.

Bear with me on that - one thing that stands out at photography club is that taking an image with same ISO, shutter speed and f/# has the 'subject' of the image at the same brightness regardless of the focal length. The focal length changes the zoom, but I use zoom loosely here, as it actually changes the "field of view", which also depends on camera sensor size, etc etc. I have cropped sensors and have tried to get away from the 'effective focal ratio' as it's not really true - my cropped sensor has a field of view less than a full frame sensor, more due to the sensor size. Yes, I know it's the "equivalent focal length" but it's a dangerous term in my view.

Coming back to the f/# - here's an interesting thing... Hubble is rated at f/24, (D=2.4m, F=57.6m) so I can take HST on with my much faster f/4.9 500mm telescope - I can take an image the same brightness as HST in 4%(ish) of the time (5 stops ish)! What I cannot match is the "zoom" where HST has ~100x more (assuming the same sensor size would remain unchanged), and as for the light pollution and atmospheric disturbances I suffer from, I don't have any real chances of competing with the HST imaging capability! HST can expose for longer without worrying about cooling (outer space is a tad cooler than a ZWO can manage!) and kicks my amateurish field of views out of the game before I even open ClearOutside to see if there is any point in getting my mount out!

Making larger aperture scopes allows imaging time to reduce ("faster", as the f/# decreases) or keep f/# same by increasing focal length, increasing "zoom/magnification/FoV" so resolving finer details. Yes, for visual the f/ratio is not seen as being as useful when we use exit pupil, but then again the exit pupil comes from the aperture anyway so a "faster" scope has a larger exit pupil for the same FOV! And we go around in circles!

I know the thing that got me was how I see the f/# now - effectively a measure of number of photons hitting sensor per unit area, and changing my camera lens focal length definition from "zoom" to "FoV for given sensor".

Edited October 6, 2021 by pete_81

[vlaiv]

6 minutes ago, pete_81 said:

Interesting thread, thought I'd add my $0.02...

Well, first - as was point out, visual is not equivalent to prime focus photography - it is equivalent to afocal method.

Second - F/ratio does not equate speed for photographic applications. Proper definition of speed would be "Aperture at resolution". In your comparison to HST - you are lacking knowledge of pixel size used for both instruments.

F/ratio really does nothing for visual - 4" F/5 scope will show the same view as 4" F/10 scope when paired with suitable eyepiece (say 10mm in F/5 with 20mm in F/10). This has been pointed out by Stu just few posts ago:

50 minutes ago, Stu said:

Speed is not really relevant for visual astronomy. At the same magnification an 8” f5 scope will show the same image size and brightness as an 8” f10.

 

[vlaiv]

Here is interesting development.

I proposed that we take extended object and place it in center of FOV and half way outside field stop - to test for integrated brightness part.

@jetstream told me that he had done so many times and there is no change in contrast/brightness perception what so ever.

[ollypenrice]

I think the thread was better off without cameras since since we've already done to death the misconceptions which abound regarding F ratio and, worse, that abomination of a term, 'crop factor.'

The matter of visual surface brightness is very involved,  even more counter-intuitive than it is involved, and is not helped by the introduction of pixels. Already we've seen the claim that Andromeda is magnified in astrophotography. Really?  There must be some big chips out there!!

Olly

[Stu]

45 minutes ago, ollypenrice said:

I think the thread was better off without cameras since since we've already done to death the misconceptions which abound regarding F ratio and, worse, that abomination of a term, 'crop factor.'

The matter of visual surface brightness is very involved,  even more counter-intuitive than it is involved, and is not helped by the introduction of pixels. Already we've seen the claim that Andromeda is magnified in astrophotography. Really?  There must be some big chips out there!!

Olly

Thanks Olly, I agree. Let’s keep this as a visual topic.

[jetstream]

2 hours ago, vlaiv said:

Here is interesting development.

I proposed that we take extended object and place it in center of FOV and half way outside field stop - to test for integrated brightness part.

@jetstream told me that he had done so many times and there is no change in contrast/brightness perception what so ever.

Zero change to my eyes - easiest to test on an object of relatively even "brightness". No as the object exits the FOV the remaining visible portion does not get dimmer to this canucks eyes.

Anyone else? what are the observations of this?

[faulksy]

11 hours ago, Stu said:

Perhaps @faulksy can give some input here, about the impact of image scale on small object visibility eg galaxies, PNs etc.

all i can say stu is. go big or go home 🤣

[Deadlake]

Interesting CN thread on aperture, magnification and exit pupil,

https://www.cloudynights.com/topic/742068-aperture-magnification-exit-pupil/

As you can see, fixing magnification x200 and changing apertures affect on what you can see:

Here is what happens when you hold Exit pupil constant while decreasing aperture.

The paradox is for an extended object no telescope will show an object with greater surface brightness than the naked eye, but we still like using a scope on it....

 

Edited October 9, 2021 by Deadlake